1. Field of the Invention
The present invention relates to the preparation of polyglycidyl ethers of polyhydroxy phenols, by means of a continuous process.
In particular, the invention relates to the reaction of an epihalohydrin with a dihydroxy phenol to form the diglycidyl ether of the dihydroxy phenol of the following formula: ##STR1## (in which O-R-O is a dihydroxy phenol group and n is a number close to zero or even zero, i.e. n usually does not exceed an average value of 0.2) in which the halogen content is reduced to a trace, especially the hydrolyzable halogen content.
The preparation of diglycidyl ethers of dihydroxy phenols will be further described with reference to the reaction of 2,2-bis (4-hydroxylphenol) propane (usually called bisphenol-A) with epichlorohydrin.
Since these diglycidyl ethers are generally called liquid epoxy resins, this terminology will be used in the description which follows.
2. Description of the Prior Art
The liquid epoxy resins form products which find many technical applications. For example, they are used in the field of varnishes and coatings in general or in the field of adhesives or agglomerates (cement or asphalt pavements).
These resins also find many applications in the electronic field (casting, printed circuits, sealing and encapsulation of electrical components) as well as in many other fields.
For certain applications, liquid epoxy resins of high purity are required, especially with regard to the content of organic or inorganic halogens, and in particular, hydrolyzable or non-hydrolyzable chlorine, which must be reduced to negligible values.
Such resins are particularly useful for special applications such as varnishes with a high resistance to chemical and atmospheric agents, and in the production of insulators and laminated electric condensers for printed circuits and for the encapsulation of electrical components. Technical processes are known for producing liquid epoxy resins from bisphenol-A and epichlorohydrin, either by a continuous or a discontinuous process, operating in the presence of an alkali metal hydroxide in quantities of 2 moles, or about 2 moles, for every mole of bisphenol-A.
In the discontinuous process, the normal procedure is to feed a concentrated acqueous solution of alkali metal hydroxide to a solution of bisphenol-A in epichlorohydrin. Such processes are, moreover, carried out at atmospheric or slightly lower than atmospheric pressure, controlling the temperature so as to distil continuously the water introduced with the alkali metal hydroxide, as an azeotropic mixture with the epichlorohydrin. When the addition of the solution of alkali metal hydroxide has been completed all the water is removed, the unreacted epichlorohydrin is recovered by distillation at pressures lower than atmospheric, and the alkali metal chloride, a sub-product of the reaction, is separated by dissolving in water.
The liquid epoxy resins obtained in this manner typically have a high viscosity, an undesirable colour, and because of their relatively high chlorine content, are not suitable for those special applications which have been previously mentioned. In fact by operating in the manner previously described, a liquid epoxy resin is obtained with a residual chlorine content of the order of 0.5 - 0.8% by weight.
Methods of producing liquid epoxy resins by a continuous process, by effecting the reaction of the bisphenol-A with the epichlorohydrin in a number of reactors installed in series are also know in the art. More particularly, in accordance with such processes, the bisphenol-A and the epichlorohydrin are continuously fed to the first of such reactors, while the alkali metal hydroxide in aqueous solution is introduced into each reactor up to a maximum quantity equal, or about equal to 2 moles for every mole of bisphenol-A. The reaction products, which are discharged continuously from the last reactor, are subjected to decantation to separate the liquid epoxy resin from the water and the alkali metal chloride which is a sub-product of the reaction.
A characteristic peculiar to these conventional processes is to carry out the reaction in the presence of oxygenated organic substances of alcoholic or ketonic nature. However, this procedure is disadvantageous, either because the presence of extraneous substances always causes a decrease in the purity of the resin produced, or because reactive substances such as the alcohols or the ketones can give rise to secondary reactions with formation of unwanted sub-products. In every case, there is the problem relating to the separation of the added substances from the liquid epoxy resin, as well as the necessity to purify such added substances before recycling them to the reaction medium.
A further disadvantage of the processes described, is the difficulty in separating the liquid epoxy resin from the water and the alkali metal halide which is a sub-product of the reaction. In fact, the difficulty and the lack of spontaneity in the separation of liquid epoxy compounds from water or aqueous saline solutions, is well known.
In order to facilitate this separation, substances capable of varying the interface tension or the density have been used in the art. However, the addition of such extraneous substances to the system causes a decrease in the purity of the resin, notwithstanding the fact that the removal of these substances often proves to be very difficult. On the other hand, when operating without these extraneous substances, lengthy periods of decantation at elevated temperature are necessary, and this normally gives rise to undesirable secondary reactions.
It is known that the formation of the liquid epoxy resins is effected by a coupling reaction between the epichlorohydrin and the bisphenol-A, followed by a dehydrochlorination reaction of the alpha-chlorohydrin thus produced.
As known, the latter is an equilibrium reaction, and this involves, at the end of the reaction, the presence of a certain quantity of free alkali metal hydroxide which promotes unfavourable secondary ramification reactions. The use, conventional in the art, of acids or acid salts to neutralize the free basicity can lead, as is known, to hazardous "interaction reactions" between the acids and the epoxy bridge of the resin.
The liquid epoxy resins produced by the described processes have a total content of hydrolyzable and non-hydrolyzable chlorine such as to make these resins unsuitable for some applications.
The quantity of non-hydrolyzable chlorine present in the liquid resin is a function of the reaction conditions and a reduction in the content of hydrolyzable chlorine can be obtained by using in the reaction medium, a quantity of alkali metal hydroxide greatly in excess of the aforesaid quantity. However, this favours the secondary reactions previously alluded to.
Moreover, it has been noted that when operating in the presence of an excess of epichlorohydrin and also possibly in the presence of added alcoholic or ketonic solvents, or other types of solvents, it was possible to reduce the content of hydrolyzable chlorine to acceptable values, solely by the use of an excess of alkali metal hydroxide such as will impair the characteristics of the liquid epoxy resin.